The invention is directed toward the field of optical communications, and more particularly, to a method and apparatus for modulating an optical communication signal using a magnetic field to adjust polarization properties.
Communication networks are used to transfer information, such as data, voice, text or video information, among communication devices, such as telephones, computer terminals, multimedia workstations, and videophones, connected to the networks. A network typically comprises nodes connected to each other, and to communication devices, by various links. Each link is typically characterized by a bandwidth or link capacity.
Most recent developments in communication technologies have been motivated by a desire to increase available bandwidth of networks to ever increasing levels. Many applications, however, do not require channels having such high bit rates. Applications that utilize an additional channel for low data rate communications, such as applications that transmit monitoring-or status information on a side channel, do not fully utilize the available bandwidth. Thus, to use a full rate channel for such low data rate communications is an inefficient use of the available bandwidth. A need therefore exists for a method and apparatus for modulating an optical communication signal at low data rates. A further need exists for a low rate communication system that supplements an existing high bandwidth system by superimposing additional lower data rate information by modulating the high bandwidth signal.
Generally, a low-rate data transmission system and method are disclosed that use the Faraday effect to modulate the polarization state of a source light traveling in an optical fiber. The resulting change in polarization is used to communicate additional information over the fiber. The low-rate data transmission system includes a low-rate signal source producing current in a wire coil wrapped around an optical fiber.
Information is conveyed in accordance with the present invention by the selective introduction of a polarization change into the light carried by the fiber. Generally, the source light at the input of the optical fiber can be randomly polarized and information is conveyed in accordance with the present invention by the presence or absence of a predefined polarization introduced into the light. A bit of information can be conveyed, for example, by selectively producing a current in the coil and introducing a predefined polarization in the light in the optical fiber.
The longitudinal magnetic field produced by a current in the coil rotates the plane of polarization of the light, in accordance with the well-known Faraday effect. When the current in the coil is modulated by the signal source, the resulting change in polarization communicates information over the fiber. According to the Faraday effect, the plane of polarization of light is rotated through an angle, "THgr", when transmitted through a material in a direction parallel to the magnetic field.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.